In plastic material transforming methods, dehumidification is a very important processing to be carried out especially before a melting step of the granular material at high temperature. During the dehumidification treatment, water included in, or otherwise associated to, the granules of the granular material, especially of the so-called “hygroscopic” granular materials, is removed. Removal of any humidity from the granular material is required, since during softening and hot melting of the plastic material water would penetrate the polymer molecular chains, which would result in chain breaking as well as in bubbles, blowholes, and both structural and colouring discontinuity being formed during cooling, thus obtaining a final transformed plastic material with poor mechanical characteristics.
Dehumidification plants for granular materials, also called dehumidifiers or dryers, have already been proposed and among these those providing the use of so-called molecular sieves are the most successful. The molecular sieves have the property of absorbing almost completely the humidity of any air passing therethrough at room temperature. The molecular sieves are contained in a suitable enclosure, called tower, provided with a humid-air inlet, on the one hand, and a dry-air outlet, on the other. The dry air coming from the molecular sieves is heated and then caused to pass through the batch of granular material to be dehumidified, and thus the humidity contained in the granular material is gradually transferred to, and moved away by, the dry and hot air. The duration of the dehumidification process depends on a number of different factors, such as the residual content of humidity, the temperature and the dry-air flow. The humidity-absorption capacity of the molecular sieves is nevertheless quantitatively limited and after a some time saturation is reached. For this reason, a bank of molecular sieves provided in suitable containers or towers, which can be connected in parallel to one another, is used, so that after saturation of the molecular sieves in one tower, the humid air is supplied to another sieve tower, whereas the saturated tower is subjected to a regeneration treatment. Thus, most dehumidification plants often operate with two towers of molecular sieves that work according to an alternating cycle. A dehumidification plant according to the prior art is disclosed in EP-1 475 593 granted to the applicant of the present invention.
Dehumidification plants known up to now, although satisfactory from many points of view, are of “rigid” type, i.e. they do not allow the dry-air flow rate to be changed in accordance with the actual use requirements, and thus treatment steps might take place in which dry heated air is supplied in excess with consequent waste of energy.